The continuous production of granules as well as coated granules by spraying liquid starting materials onto or into the fluidized bed of a one-stage or multi-stage fluidized bed reactor is becoming increasingly technically important. An overview of the technology of fluidized bed spray granulation that is suitable for both the production and coating of granules, is provided by H. Uhlemann in Chem. Ing. Tech, 62 (1990), pp. 822-834. Fluidized bed reactors generally comprise a vessel with a fluidizing base plate arranged horizontally or inclined therein, the region above the fluidizing base plate being termed the fluidized bed chamber and the region underneath the floor being termed the blast box, devices for introducing a fluidizing gas, which may at the same time serve as a drying and/or reaction gas, and for removing the gas leaving the fluidized bed, devices for introducing solid starting substances such as nuclei or particles to be coated, and/or introducing liquid starting materials such as solutions and melts, as well as one or more devices for discharging the granules formed in the fluidized bed.
The discharge devices may comprise those with or without a screening action. Devices with a screening-type granule discharge generally include screening pipes whose upper end is situated in the plane of the fluidizing base plate and through which flows a screening gas in the direction opposite to the granule discharge. A non-screening discharge may also be arranged as an outlet in the side wall of the fluidized bed chamber, the lower edge of the outlet opening being at the level of the fluidized bed floor. According to EP Patent 0 332 929 a zigzag screening device is joined to the fluidizing base plate of a fluidized bed granulator for the fluidized bed spray granulation and a feedback shaft is mounted at the connection site. In order to avoid blockage the granules are fed from the fluidized bed chamber through an underflow weir to the screening device.
In the continuous production of granules by fluidized bed spray granulation and coating of these granules by spraying the latter with a liquid coating material in a following second fluidized bed reactor, the granules from the first fluidized bed reactor were discharged downwards through the floor via screening pipes and were then passed by means of blow-through chamber wheel locks and pneumatic conveying lines to the second fluidized bed reactor serving as the coating reactor. It was found that the operating costs and the susceptibility to breakdown of this procedure are high. In order to reduce expenditure on apparatus and power and labor costs, attempts have been made to carry out both stages in a continuous operation by using a tray-shaped fluidized bed reactor comprising two zones. Conventional weirs with and without underflow openings were tested as a device for separating the two zones. In all cases unallowable back-mixing occurred. A further problem was that the filling height, which is very important as regards product quality and operational reliability, cannot be maintained constant. Fluctuating fluidized bed heights also had a deleterious effect on the service life of the spray nozzles.